In the case of the potentiometric measuring point for determining the ion concentration in a liquid medium, for instance, a pH sensor. The pH sensor can be embodied as a glass electrode or as an ISFET sensor. The voltage which forms between the measuring half cell and the reference half cell serves as a measure for the pH value, or for the ion concentration of the medium. The fundamentals of pH measurement technology and the construction of pH sensors are described, for example, in the book “Abwasser-Meβ-und Regeltechnik” (Wastewater-Measuring and Control Technology), Publisher: Endress+Hauser GmbH+Co., 2nd Ed., Pgs. 81 et seq.
Preferably, the pH-measuring half cells are so-called glass electrodes or ISFET sensors. These find broad application in many branches of chemistry, environmental testing, medicine, industry and water management. Both types of sensors are available from the assignee for the most varied of applications. As already indicated, the glass electrodes and ISFET sensors used for potentiometric measurements are commonly combined with reference half cells, which exhibit highly constant potentials.
In the case of glass electrodes, silver/silver-chloride or calomel electrodes are, as a rule, used. The contact of the reference half cell with the medium being measured is produced by way of bridge electrolytes. The bridge electrolyte can be liquid or solid and must, as a rule, fulfill certain prerequisites: On the one hand, it should have little influence on the potential of the reference half cell; on the other hand, it should form with the medium being measured a smallest possible diffusion potential. Provided that the prerequisites are fulfilled, the reference half cell provides a process-independent and stable reference signal.
In many instances of application of pH, REDOX and ISE measurement technology, liquid-bridged reference half cells are used. Liquid-bridged reference half cells use a liquid contact between the process—i.e. the medium—and the interior of the reference half cell. This liquid contact is usually provided in the form of a porous ceramic rod with a pore diameter in the μm-range. Now, process factors can lead to a plugging of this porous ceramic. If a plugging or blocking of the ceramic occurs, the junction assumes a very high resistance and no longer provides a low-resistance coupling of the reference half cell to the medium. Consequently, disturbance voltages can become superimposed on the potential of the reference half cell, and these can, among other things, significantly compromise the accuracy of measurement. In the case of a pH-value measurement, these disturbance voltages can even correspond to changes of multiple pH-values. As a result of the disturbance voltages, the measuring point then outputs pH-values no longer reflecting the actual ion concentration in the medium. In practice, moreover, about 90% of the bad measurements occurring in the case of ion concentration measurements are caused by a malfunctioning of the reference half cell.
A method does already exist for recognizing a malfunctioning of a reference half cell caused by the blocking of the junction between the reference half cell and the medium being measured. According to this known method, a malfunctioning of the reference half cell is recognized by monitoring in the process the impedance of the liquid junction between the reference half cell and the medium being measured. As soon as a predetermined limit value is exceeded, an alarm is activated.
FIG. 1 shows the essential components of a pH measuring point 1, as is used in measurement technology. The measurement point 1 includes a measuring half cell 2, a reference half cell 3 and a measuring device 6, which usually measures the voltage between the two half cells 2, 3. This voltage is inversely proportional to the pH-value of the medium 7 being measured.
The pH-measuring half cell 2 usually has an internal resistance of 50 to 1000 M. Via the medium 7 being measured, there is a connection to the liquid-bridged reference half cell. This connection usually has an impedance in the order of magnitude of 1-100 k and, therefore, lies at a few orders of magnitude below the impedance of the measuring half cell 2. The measuring device 6 determines the voltage between the two half cells 2, 3, with the reference half cell 3 lying at ground potential in the measuring device. Due to the relatively low impedance of the liquid-bridged reference half cell 3, the medium 7 is, consequently, also at the ground potential, up to the glass membrane. If a blockage of the liquid-bridged reference half cell 3 arises, then electrical disturbance potentials between the measuring half cell 2 and the reference half cell 3 become noticeable in the measuring. Since the measuring half cell 2 and the reference half cell 3 are, considered electrically, connected in series, the sum of the impedances is dominated by the impedance of the measuring half cell 2. For this reason, as illustrated in FIG. 1, a simple resistance measurement between the points I and II does not allow any conclusion as to the impedance of the reference half cell 3 at the moment.
In order to achieve a targeted monitoring of the impedance of the reference half cell 3, it is known to use a symmetrically-connected measuring point 1. A circuit of this type is displayed schematically in FIG. 2. The measuring half cell 2 is operated at low resistance relative to a metal rod 10; the reference half cell 3 is also measured relative to the metal rod 10. The metal rod 10 has the advantage, as compared to the reference half cell 3, that it does not get blocked. It is true that the metal rod 10 does not deliver a constant reference potential, since redox potentials can develop on it. This is, however, not of concern for the measurements by means of the measuring devices 8 and 9, since, in the end, the difference of the measured values from the two measurements is formed, so that the influence of the changing redox potentials on the metal rod 10 drops out. Consequently, the impedance measured between the two points I and II depends essentially on the impedance of the liquid-bridged reference half cell 2. Therefore, this method is ideally suited for recognizing a malfunctioning of the reference half cell 3 due to blocking.
The disadvantages of this known solution are, however, not to be overlooked:                A not insignificant extra burden has to be carried. Along with the extra metal rod, there is a more complicated suspension system, extra cable, and an expanded electronics.        An alarm indicating malfunctioning of the reference half cell is first triggered, after an earlier established limit value is exceeded. The alarm is activated completely independently of whether the increased value of the impedance of the reference half cell is, in fact, even affecting the measurement or whether the disturbance was perhaps already so grave, even before the reaching of the limit value, that the measurement was already at that time significantly affected.        